Gastric adenocarcinoma is characterized by an extremely virulent behavior and for which the mortality approximates the incidence (see, Alexander, et al. CANCER OF THE STOMACH IN CANCER: PRINCIPLES AND PRACTICE OF ONCOLOGY, DeVita, et al. (Eds.), J. B. Lippincott Co., Philadelphia, Pa., p. 818-848 (1993)). The vast majority of patients with gastric cancer are diagnosed with advanced stage disease and even after "curative" gastrectomy, most will die from recurrent disease. Recently, there has been increasing interest in the use of neoadjuvant or primary chemotherapy, frequently using fluoropyrimidine-based combination chemotherapy, in an attempt to increase resectability and improve survival for patients with locally advanced gastric cancer. Neoadjuvant chemotherapeutic treatment provides an early opportunity to assess individual patient response using the in situ primary tumor. Overall response rates in studies using neoadjuvant 5-fluorouracil (5-FU) based regimens in locally advanced gastric cancer range from 24 to 45% (see, Ajani, et al., Cancer 68:1501-1506 (1991); Leichman, et al., J Clin Oncol 10:1933-1942 (1992); and Ajani, et al., Proc ASCO 11:165 (1992)). Therefore, at least half of all patients treated in this setting are being subjected to unnecessary toxicity and delay in operation with no therapeutic benefit. Strategies that would accurately predict tumor responsiveness to 5-FU therapy would provide an opportunity to selectively treat patients most likely to benefit from treatment and avoid unnecessary toxicity in those who would not.
The fluoropyrimidines are an important group of antineoplastic agents used widely in the treatment of gastrointestinal as well as other tumors. An important mechanism of 5-FU cytotoxicity is through the inhibition of thymidylate synthase (TS), the enzyme that catalyzes the methylation of 2'-deoxyuridine-5'-monophosphate (dUMP) to 2'-deoxythymidine-5'-monophosphate (dTMP), an essential step in DNA biosynthesis. The level of TS within an experimental tumor has been shown to correlate with 5-FU sensitivity; further, the acute overexpression of TS after 5-FU exposure is associated with resistance to the cytotoxic effects of the drug (Spears, et al., Cancer Res 42:450-456 (1982); Keyomars, et al., J Biol Chem 263:14402-14409 (1988); Berger, et al., Mol Pharmacol 28:461-467 (1985); Clark, et al., Cancer Treat Rep 71:261-265 (1987); and Swain, et al., J Clin Oncol 7:890-899 (1989)). Recently, TS monoclonal antibodies have been developed and characterized that are highly specific and sensitive (K.sub.d range 0.3-11.0). One of these antibodies, TS106, can detect femtomolar concentrations of cytosolic free (Mr 36,000) and complexed (Mr 38,000) TS enzyme by Western immunoblot analysis (see Johnston, et al., Cancer Res 51:6668-6676 (1991) and Johnston, et al., Cancer Res 52:4306-4312 (1992)).
We have conducted a study of neoadjuvant 5-FU, leucovorin, and interferon-.alpha. followed by resection and consolidation therapy in 22 patients with locally advanced gastric adenocarcinoma. Tumor specimens were obtained endoscopically in 13 patients prior to treatment and during the second or third day of cycle 2 for TS protein quantification and to assess the relationship between clinical response of the primary tumor and patient survival with free, total, or complexed TS protein expression before and after exposure to 5-FU.